What are the key clinical features that suggest a diagnosis of Legionnaires' disease in a patient presenting with pneumonia?

Key clinical features suggesting Legionnaires' disease in a patient with pneumonia include:

• A severe pneumonia presentation often accompanied by high fever, dry cough, and dyspnoea, which may be more pronounced than in typical pneumonias NICE CG191 Cunha 2010.
• Extrapulmonary manifestations such as headache, confusion or other neurological symptoms, gastrointestinal symptoms including diarrhoea, nausea, and vomiting, which are more common in Legionella infection than in other pneumonias NICE CG191 Johnson & Cunha 1993.
• Relative bradycardia (pulse-temperature dissociation), where the heart rate is lower than expected for the degree of fever, is a distinctive clinical clue NICE CG191 Fairbank et al. 1991.
• Laboratory findings supporting Legionnaires' disease include hyponatraemia, elevated liver enzymes, and raised creatine kinase, which help differentiate it from other causes of pneumonia NICE CG191 Cunha 2010.
• Radiologically, Legionella pneumonia may show patchy or segmental infiltrates that can rapidly progress, sometimes with pleural effusions NICE CG191 Fairbank et al. 1991.

These features combined—severe pneumonia with systemic and extrapulmonary symptoms, relative bradycardia, characteristic lab abnormalities, and radiological findings—should raise clinical suspicion for Legionnaires' disease and prompt specific diagnostic testing and management NICE CG191 Cunha 2010.